In a multicarrier system, a communication path having a fixed bandwidth is divided into a number of sub-bands having different frequencies. The width of the sub-bands is chosen to be small enough to allow the distortion in each sub-band to be modeled by a single attenuation and phase shift for the band. If the noise level in each band is known, the volume of data sent in each band may be optimized by choosing a symbol set having the maximum number of symbols consistent with the available signal to noise ratio of the channel. By using each sub-band at its maximum capacity, the amount of data that can be transmitted in the communication path is maximized.
In practice, such systems are implemented by banks of digital filters which make use of fast Fourier transforms. Consider the case in which a single data stream is to be transmitted over the communication path which is broken into M sub-bands. During each communication cycle, the portion of the data stream to be transmitted is converted to M symbols chosen to match the capacity of the various channels. Each symbol is the amplitude of a corresponding sub-carrier. The time domain signal to be sent on the communication path is obtained by modulating each sub-carrier by its corresponding amplitude and then adding the modulated carriers to form the signal to be placed in the communication path. This operation is normally carried out by transforming the vector of M symbols via the inverse Fourier transform to generate M time domain values that are sent in sequence on the communication path. At the other end of the communication path, the M time domain values are accumulated and transformed via a Fourier transform to recover the original M symbols after equalization of the transformed data to correct for the attenuation and phase shifts that occurred in the channels.
This idealized system encounters two types of problems in practice. First, in many environments, the noise encountered is restricted to a few narrow frequency sub-bands; however, the noise has an amplitude that is of the same order, or even greater, than the signals sent in the sub-band. This type of noise results from the pickup of other narrow band communication signals that impinge on the communication path. These signals enter the system at points in the communication that are not sufficiently shielded. In long communication paths, providing perfect shielding is not practical.
In principle, a multicarrier transmission system can detect the presence of a high noise signal in one sub-band and merely avoids transmitting data in that sub-band. In practice, this solution does not function properly because of the characteristics of the sub-bands obtained using Fourier transforms. The Fourier transform provides sub-bands that are isolated by only 13 dB. Hence, the sub-bands have sidelobes that extend into the neighboring channels. A large noise signal in one channel will spill over into several channels on each side of the channel in question. Hence, a substantial fraction of the communication path capacity may need to be taken off line to avoid a high intensity, narrow band noise signal.
The second type of problem is encountered in multi-point transmission systems. Consider the case in which a number of subscribers are located along a communication path which couples each subscriber to a central office. In the simplest case, each subscriber is assigned a first sub-band to send messages to the central office and a second sub-band to receive messages from the central office. The multicarrier system described above assumes that all of the subscribers and the central office are synchronized with one another. If a subscriber is out of synchronization with the central office, intersymbol interference can occur. That is, the symbol decoded by the subscriber will include interference from other symbols in other sub-bands and/or earlier or later symbols transmitted in the subscriber's sub-band. This type of interference is further aggravated by the high sidelobes in the sub-bands provided by the Fourier transform.
Prior art systems solve the intersymbol interference problems by including additional data in each sub-band that can be used to correct the timing errors. This additional data reduces the amount of information that can be transmitted on each sub-band, since the data must be sent in the sub-band, and hence, uses some of the bandwidth of the sub-band.
Broadly, it is the object of the present invention to provide an improved multi-carrier transmission system.
It is a further object of the present invention to provide a multi-carrier transmission system that is better adapted to the elimination of narrow band noise signals than prior art systems.
It is a still further object of the present invention to provide a multi-carrier transmission system in which intersymbol interference resulting from timing errors may be eliminated without reducing the bandwidth of the communication channels.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.